The present disclosure relates generally to a low pressure mercury vapor discharge lamp and more particularly to a compact fluorescent lamp including an auxiliary amalgam for emitting mercury vapor during at least a starting period.
A wide variety of low-pressure discharge lamps are known in the art. Low pressure mercury vapor discharge lamps have a maximum efficiency of converting supplied electrical energy into ultraviolet radiation at an optimal mercury vapor pressure. The mercury vapor pressure is typically very highly dependent on the operating temperature of the lamp. Compact fluorescent lamps, which have bent tubes forming convoluted discharge paths, typically have high loads at the walls and therefore high temperatures are reached at the wall during operation of the lamp, typically about 70 to about 140 degrees Centigrade (C). At these high temperatures the vapor pressure of the mercury can increase above the optimal.
To control the mercury vapor pressure near the optimal level, an amalgam is used in place of conventional liquid mercury. As the mercury vapor pressure in the lamp increases to an undesirable level, the amalgam begins to melt and form a solution with mercury vapor to decrease the mercury vapor pressure in the lamp back toward the optimal level. The location of the amalgam, which has a predetermined melting temperature, it important in providing the desired improvement because the location of the amalgam affects its temperature during operation of the lamp. The amalgam typically used in areas near high temperature walls is bismuth-indium-mercury (Bi—In—Hg).
Lamps using an amalgam optimized for use in high temperature areas have the disadvantage of a longer warm-up or starting period than lamps suing pure liquid mercury. The length of the starting period is dependent on the speed at which the mercury vapor pressure in the lamp increases because the lumen output of the lamp is dependent on the mercury vapor pressure in the lamp. The starting period is longer for amalgam containing lamps because the mercury vapor pressure is too low at lower temperatures usually present at start-up, typically in the range of about 0 degrees C. to about 50 degrees C. The mercury vapor pressure increases slowly and doesn't reach its proper level until the amalgam reaches the high temperatures. In contrast, the mercury vapor pressure of a liquid mercury dosed lamp is much higher than the mercury vapor pressure of the amalgam containing lamp at the lower temperature or at room temperature.
To improve warm-up characteristics of an amalgam containing lamp, an auxiliary amalgam is typically attached to each electrode stem so that the auxiliary amalgam emits mercury during the starting period. The auxiliary amalgam is heated by the cathode after ignition and emits mercury vapor to make up for the lack of mercury vapor during the starting period. The auxiliary amalgam typically used is indium-mercury (In—Hg). The amalgam which controls the mercury vapor pressure during operation, except for the starting period, is typically called the main amalgam, in contrast with the auxiliary amalgam which controls the mercury vapor pressure during the starting period.
Amalgams containing low pressure mercury vapor discharge lamps have experienced varying degrees of success. Thus, a need exists for an improved low-pressure mercury vapor discharge lamp having improved warm-up characteristics.